1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same. More particularly, the present invention relates to an optical scanning device that is suitably used for an apparatus such as a laser beam printer or a digital copying machine having an electrophotographic process, in which a light flux optically modulated and emitted from a light source unit is reflected and deflected on a polygon mirror serving as an optical deflection unit and then a surface to be scanned is scanned with the light flux through a scanning optical system to record image information. In addition, the present invention relates to a color image forming apparatus which uses a plurality of optical scanning devices and is composed, of a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
Up to now, in an optical scanning device used for a laser beam printer (LBP) or the like, a light flux optically modulated according to an image signal and emitted from a light source unit is periodically deflected by, for example, an optical deflector composed of a rotating polygonal mirror (polygon mirror). The deflected light flux is converged to form a spot shape on a photosensitive recording medium (photosensitive drum) by a scanning optical system having an fθ characteristic. The surface of the recording medium is scanned with the light flux to perform image recording.
FIG. 7 is a schematic view showing a main part of a conventional optical scanning device.
In FIG. 7, a divergent light flux emitted from a light source unit 1 is converted into a substantially parallel light flux by a collimator lens 3. The substantially parallel light flux is limited by a diaphragm 2 and incident into a cylindrical lens 4 having predetermined refractive power only in the sub scanning direction. Of the substantially parallel light flux incident into the cylindrical lens 4, the light flux within the main scanning section is exited without changing an optical state. The light flux within the sub scanning section is converged and imaged as a substantial linear image onto a deflection surface (reflection surface) 5a of a deflection unit 5 composed of a polygon mirror.
The light flux which is deflected on the deflection surface 5a of the deflection unit 5 is guided onto a photosensitive drum surface 9 serving as a surface to be scanned through a scanning optical system 6 having an fθ characteristic. By rotating the deflection unit 5 in a direction indicated by an arrow “A”, the photosensitive drum surface 9 is scanned with the light flux in a direction indicated by an arrow “B” to record image information.
Further, in order to achieve high speed scanning, a multi-beam optical scanning device that simultaneously forms a plurality of scanning lines by light fluxes from a plurality of light sources has been proposed and commercially available from various companies. FIG. 8 is a schematic view showing a main part of a multi-beam optical scanning device. Two light fluxes emitted from light sources 81 and 82 are converted into parallel light fluxes by collimator lenses 83 and 84 and then synthesized into one by a synthesizing optical element 85. The synthesized light flux forms a linear image extended in the main scanning direction near a deflection surface of a polygon mirror 87 by the action of a cylindrical lens 86 and then forms a light spot on a photosensitive drum 89 by a scanning optical system 88. Therefore, the two scanning lines can be formed by performing optical scanning once, so that extremely high speed scanning can be achieved as compared with a conventional optical scanning device. With respect to a multi-beam light source other than one using the above-mentioned synthesizing optical element, a monolithic multi-beam laser in which a large number of light emitting points exist has been produced. In the case where the monolithic multi-beam laser is used, it is unnecessary to use the synthesizing optical element. Thus, it is possible to simplify the optical system and the optical adjustment.
A semiconductor laser used as a conventional light source (for example, Japanese Patent Application Laid-Open No. H9-021944) is an infrared laser (780 nm) or a visible laser (675 nm). However, in order to realize a high resolution, the development of an optical scanning device in which a minute spot shape is obtained by using a short wavelength laser having an oscillating wavelength of 500 nm or less is under way. The advantage of the use of the short wavelength laser is that a minute spot size which is about half of a conventional spot size can be achieved while an exit F number of the scanning optical system is kept equal to a conventional one. In the case where a spot size is reduced to half of the conventional spot size while using the infrared laser, it is necessary to increase the intensity of the scanning optical system to about two times larger than that in a conventional case. A focal depth is proportional to a wavelength of a used light source and to the square of the exit F number of the scanning optical system. Therefore, to obtain the same spot size, the focal depth in the infrared laser becomes equal to or smaller than about ½ of the focal depth in the short wavelength laser.
In such an optical scanning device, in order to record image information with high precision, it is necessary to preferably correct a curvature of an image plane over the entire surface to be scanned, to have a distortion characteristic (fθ characteristic) related to a uniform speed between an angle of view θ and an image height Y, and to make spot sizes on the image plane uniform at respective image heights. Various optical scanning devices or various scanning optical systems that satisfy the optical characteristics like those have been proposed up to now.
In particular, in the optical scanning device using the multi-beam light sources, in order to prevent a jitter (variations in scanning lines on the photosensitive drum surface in the main scanning direction) resulting from a difference of wavelengths among the plurality of light sources, a measure in which the light sources are selected so as to minimize the difference of wavelengths among the light sources have been taken. In the case where the jitter resulting from the difference of wavelengths among the light sources (chromatic aberration of magnification) is corrected by the scanning optical system, as disclosed in Japanese Patent Application Laid-Open No. H9-021944, a plurality of glass lenses having different dispersion characteristics are required. Therefore, as compared with a scanning optical system in which no chromatic aberration of magnification is corrected, generally, the number of lenses increases, thereby increasing the cost. In addition, there is a limitation with respect to the selection of wavelengths of the light sources, so that it is hard to completely make the wavelengths uniform. An increase in the cost required for the selection of the wavelengths also becomes a problem.
Further, when the semiconductor laser is activated, an image quality reduces due to a variation in wavelength which is called a mode hopping. Thus, even in an optical scanning device other than the optical scanning device using the multi-beam light sources, in order to improve the stability of the image quality, it is necessary to minimize the jitter caused by the variation in wavelength.
Further, as compared with the case where the infrared laser is used, a dispersion of the optical material becomes larger in the case of the high precision optical scanning device in which the wavelength of the light source is shortened, and this becomes a problem. FIG. 9 is a sectional view showing a main part of a general optical scanning device using two plastic lenses (see Table 3 with respect to design values). A light beam emitted from the light source 1 is converted into substantially parallel light by the collimator lens 3. Then, the parallel light is temporarily imaged to the vicinity of the reflection surface 5a of the deflection unit 5 in the sub scanning direction by the cylindrical lens 4. The light beam which is deflected and reflected on the polygon mirror 5 is scanned at constant speed by the two refractive lenses 7 and 8 and imaged to a minute spot on the surface to be scanned 9. FIG. 10 is a graph showing a calculation result of chromatic aberration of magnification with respect to the infrared laser (780 nm) used as the conventional light source and a purple-blue color laser (408 nm) used for the high resolution optical scanning device in the case where the optical scanning device is used. FIG. 10 is a plot of differences between imaging positions in the main scanning direction in the case where a difference of wavelength is set to 5 nm and an imaging position in the main scanning direction at a reference wavelength for each image height (for example, a difference between an imaging position at 785 nm and an image position at 780 nm). In the optical system using the two plastic lenses made of the same material, the chromatic aberration of magnification cannot fundamentally be corrected. Up to now, the dispersion characteristic of the material is in a level in which a problem is not really caused because the oscillating wavelength of the laser is relatively long. Therefore, even in the case where the chromatic aberration of magnification is not corrected by the scanning optical system, the jitter can be reduced by the measure such as the selection of the light sources. However, in the case where an optical system of the same type as the one using the two plastic lenses is used for the short wavelength laser, the dispersion characteristic of the material is deteriorated four times to eight times (see FIG. 11), so that the chromatic aberration of magnification of about 70 μm is caused in the circumference of an image. This corresponds to about 1.6 pixels in an image forming apparatus having 600 dpi. Thus, the correction of the chromatic aberration of magnification is an essential prerequisite in an optical scanning device using the short wavelength laser having 500 nm or less. However, because the number of materials having the dispersion characteristics is small for the plastic lenses, the chromatic aberration of magnification cannot be corrected using only the plastic lenses.
TABLE 1(Conventional Example)Design dataWavelength, RefractiveLens 7 (Figure)indexUse wavelengthλ (nm)408First planeSecond planeLens 7 Refractive indexnd1.53064R−8.16372E+01−4.45500E+01νd55.50K−1.55555E+00−5.06325E−01Lens 8 Refractive indexnd1.53064B4  6.47801E−08  3.12584E−07νd55.50B6  1.11313E−09  2.34564E−10Light beam angleB8−3.11807E−12−2.40882E−13Incident angle to polygonθp−70B10  1.20455E−15−7.09973E−16Maximal exit angle onθe45polygonArrangementLens 8 (Figure)Polygon surface to lens 7e125First planeSecond planeCentral thickness of lens 7e211R−3.60006E+02∞Lens 7 to lens 8e377K−4.13148E+01Central thickness of lens 8e45B4  2.31574E−07Lens 8 to surface to beSk105.36476B6−2.28750E−11scannedPolygon axis to surfaceL229.78B8  1.24904E−15to be scannedEffective scanning widthW297B10−2.71574E−20